Route dependents to the selected file with pulsing magenta dashes

Files that import the selected file now get street routes too, from
the same single search (paths cost the same both ways), reversed so
they flow toward the selection. They draw as magenta dashes marching
inward with a pulse travelling along them, dashed on the minimap too.
Ribbon geometry moves into city.js and every ribbon keeps to the right
of its direction of travel, so an import and a dependent sharing a
street run in separate lanes instead of overlapping (tested).

README.md

@@ -34,7 +34,8 @@ Configuration:
   - `T` or **Tour** flies over the largest districts. **Copy link** shares the repository, view,
     camera and selected file as a URL fragment (GitHub repositories only).
   - Touch: left thumb joystick, right thumb look, pinch to zoom.
-  - Selecting a file also lays glowing routes along the streets to each file it imports.
+  - Selecting a file also lays glowing routes along the streets: cyan/amber chevrons out to each
+    file it imports, and pulsing magenta dashes in from each file that imports it, in separate lanes.
   - Residents wander the streets (toggle in Settings). A wall with posters encloses the city.
   - Signed in with GitHub, files with open code scanning or Dependabot alerts smoke (and burn
     for critical/high) and are wrapped in hazard tape. The GitHub App needs read access to